Large storage tanks are often cylindrical and have a circular floating roof. The roof floats on the surface of the liquid, thereby decreasing the vapor space inside of the tank. A floating roof may be required for reasons of safety or for pollution reduction. The floating roof has a seal between its outer edge and the wall of the tank that helps to prevent the escape of the contained liquid or vapors from that liquid. This seal moves up and down with the roof as the liquid level changes.
There are two broad types of storage tanks that utilize floating roofs: tanks having an exposed floating roof and tanks having a fixed roof covering the floating roof. An advantage of the covered tank is that it protects the floating roof from undesirable effects from the external environment, such as rain or birds. A disadvantage of the covered tank is that volatile, explosive, corrosive, or toxic gasses or liquids can accumulate between the floating roof and the fixed roof.
When the contained liquid or its vapors are flammable, a substantial fire hazard can exist at the roof of the tank. The fuel for such a fire can be from the escape of liquid or vapor from the storage tank. The industry is therefore quite interested in monitoring systems that can be used to improve safety by identifying fires or any excessive heating conditions that may lead to fire.
The linear heat detector (LHD) is an existing technology that is typically realized using one of two configurations. With the first configuration, referred to as a “digital” LHD, a cable containing two wires is installed around the circumference of the tank between the primary seal and the outer seal of the tank. The digital LHD cable comprises two insulated steel conductors, which may be copper-coated. The insulated conductors are twisted in a helical configuration resulting in a residual spring-like stress in the cable. If the ambient temperature reaches the melting point of the insulating material, the conductors push their way through the insulation, thereby contacting one another and short-circuiting the two conductors. This short-circuit condition is detected by measuring the current flow through the LHD, thereby raising an alarm condition. When such a short-circuit condition occurs, it is necessary to replace the affected part of the LHD to restore normal operation.
The second LHD configuration, known as “analog” LHD, uses four wires: one pair of copper wires carries a reference current through the LHD loop. The second pair of wires is coated with a negative-temperature-coefficient (NTC) insulation. Elevated temperatures are detected by comparing the reference current to the current in the NTC-insulated loop. This approach has the advantage of supporting the estimation of loop temperature. This NTC-based system will continue to operate normally if an elevated temperature is subsequently reduced, provided that the elevated temperature was not high enough to permanently damage the cable.
In the current state-of-the-art, a lengthy umbilical cable is used to connect the LHD on the floating tank roof to the monitoring system connection that is located near the top of the tank. This umbilical cable is subjected to stresses from self-weight and stresses during tank movement or adverse weather conditions. It may subsequently fail, resulting in a loss of monitoring capability and necessitating costly repairs.